Process for reducing ultra low sulfur diesel color

ABSTRACT

The present invention is a process for preparing ultra low sulfur diesel. The steps include reacting a feedstock of petroleum crude oil with hydrogen in the presence of a hydrodesulfurization catalyst under hydrodesulfurization conditions, fractionating the reaction products, flash distilling the bottoms fraction, condensing the volatile distillate fraction as ultra low sulfur diesel, and recycling the distillation bottoms fraction for further reacting with hydrogen.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit and priority of U.S. Provisional PatentApplication No. 61/671,484 filed on Jul. 13, 2012, which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to processing Ultra Low Sulfur Diesel (ULSD). Inparticular, the invention relates to hydrodesulfurizing ULSD andreducing the color of the ULSD.

DESCRIPTION OF RELATED ART

Ultra Low Sulfur Diesel must satisfy regulatory and industry standards.Some of those standards relate to impurities such as sulfur. Othersrelate to physical properties such as flash point. Yet, other standardsrelate to manufacturing controls.

With regard to sulfur impurities, hydrocarbon fractions such as dieselfuel produced in the petroleum industry are typically contaminated withvarious sulfur-based impurities. The presence of sulfur compounds isundesirable since they result in a serious pollution problem. Combustionof hydrocarbons containing these impurities results in the release ofsulfur oxides which are noxious and corrosive.

Federal legislation, specifically the Clean Air Act of 1970 and its 1990Amendments (42 U.S.C. §7401 et seq. (2008)), has imposed increasinglymore stringent requirements to reduce the amount of sulfur released tothe atmosphere. Under its rule-making authority, the United StatesEnvironmental Protection Agency has lowered the sulfur standard fordiesel fuel to 15 parts per million by weight (ppm or μg/g).

ASTM International, formerly known as the American Society for Testingand Materials (ASTM), has established test methods for measuringimpurities such as sulfur and physical properties such as flash point.ASTM International sets forth a Standard Specification for Diesel FuelOils in ASTM D975-08. Notably, the Specification covers seven grades ofdiesel fuel oils for various types of diesel engines.

With regard to Grade No. 1-D S15 and Grade No. 2-D S15 (bothspecial-purpose, light middle distillate fuel for use in diesel engineapplications requiring a fuel with 15 ppm sulfur (maximum)), ASTM D975sets forth alternate test methods (i) ASTM D2622 “Standard Test Methodfor Sulfur in Petroleum Products by Wavelength Dispersive X-rayFluorescense Spectrometry” for measuring sulfur in the range of 0.0003to 5.3 mass %, (ii) ASTM D3120 “Standard Test Method for TraceQuantities of Sulfur in Light Liquid Petroleum Hydrocarbons by OxidateMicrocoulometry” for measuring sulfur in the range of 3.0 to 100 mg/kg(wt ppm), and (iii) ASTM D5453 “Test Method for Determination of TotalSulfur in Light Hydrocarbons, Spark Ignition Engine Fuel, Diesel EngineFuel, and Engine Oil by Ultraviolet Fluorescence” for measuring sulfurin the range of 0.0001 to 0.8 mass % or 1.0 to 8000 mg/kg (wt ppm).

Other properties specified under ASTM D975 include flash point, waterand sediment, distillation temperature, kinematic viscosity, and others.

While the Clean Air Act, its related rules, or the ASTM InternationalStandard Specifications do not set forth a color requirement for ULSD,determination of the color of petroleum products has been used formanufacturing control purposes. Furthermore, it is an important qualitycharacteristic because color is readily observed by the user of theproduct. ASTM D1500 “Standard Test Method for ASTM Color of PetroleumProducts (ASTM Color Scale) and ASTM D6045 “Standard Test Method forColor of Petroleum Products by the Automatic Tristimulus Method” areappropriate methods for measuring color of ULSD. A commonly-appliedmanufacturing control limit is 2.5 ASTM.

The manufacturing of ULSD presents challenges in satisfying regulatoryand industry standards. Notably, the Clean Air Act and its related rulesensure that there is a continual need for more effective desulfurizationmethods. Moreover, the deterioration in ASTM Color does not correlateperfectly to the most common desulfurization method.

The most common method of desulfurization of fuels ishydrodesulfurization, in which the fuel is catalytically reacted withhydrogen gas at elevated temperature and high pressure in the presenceof a costly catalyst. For example, U.S. Pat. No. 5,985,136 describes ahydrodesulfurization process to reduce sulfur level in napthafeedstreams. Organic sulfur is reduced by this reaction to gaseous H₂S,which is then oxidized to elemental sulfur by the Claus process.

With this method, it is believed that the catalyst has several months ofremaining life for reducing the sulfur level even though the ASTM Colorbegins to exceed 2.5. It is desirable to replace the catalyst only afterit is fully exhausted in the process, not as prematurely required underthe current process. There is a need to improve the process so that theASTM Color does not exceed 2.5 until the catalyst is fully exhaustedwith regard to hydrodesulfurization.

To cause the decay in catalytic reduction of the sulfur level tocorrelate more closely to the deterioration in achieving the desiredASTM Color, the inventor in copending U.S. patent application Ser. No.13/156,487 teaches installation of a high-efficiency, flash tank at theend of the hydrotreating process. The flash tank's efficiency yields anoverhead stream that 99+% ULSD and a small bottom stream containing thecolor bodies.

It remains desirable to provide a process that does not require a flashtank with a total stage efficiency of greater than 99%.

SUMMARY OF THE INVENTION

The present invention is a process for preparing ultra low sulfurdiesel. The steps include reacting a feedstock of petroleum crude oilwith hydrogen in the presence of a hydrodesulfurization catalyst underhydrodesulfurization conditions, fractionating the reaction products,flash distilling the bottoms fraction, condensing the volatiledistillate fraction as ultra low sulfur diesel, and recycling thedistillation bottoms fraction for further reacting with hydrogen.

The present invention provides an improved process for preparing ultralow sulfur diesel, wherein the hydrodesulfurization catalyst is morefully utilized while retaining suitable color.

DESCRIPTION OF THE DRAWINGS

Further details will be apparent from the following detaileddescription, with reference to the enclosed drawing, in which:

FIG. 1 is a block flow diagram of a process according to the presentinvention.

DETAILED DESCRIPTION

In an embodiment, the present invention is a process for preparing ultralow sulfur diesel which is diesel having a sulfur content of less than15 parts per million. The process steps include (a) reacting a feedstockof petroleum crude oil with hydrogen in the presence of ahydrodesulfurization catalyst under hydrodesulfurization conditions, (b)fractionating the reaction products into a naphtha fraction, a kerosenefraction, and a first bottoms fraction, (c) flash distilling the firstbottoms fraction in a distillation column having at least one stage anda total stage efficiency of less than about 99%, thereby yielding avolatile distillate fraction and a second bottoms fraction, (d)condensing the volatile distillate fraction as ultra low sulfur diesel,and (e) recycling the second bottoms fraction into the feedstock ofpetroleum crude oil for further reacting of the second bottoms fractionwith hydrogen in the presence of the hydrodesulfurization catalyst. Thevolatile distillate fraction should have an ASTM Color of less than orequal to 2.5 and a sulfur content less than 15 ppm. More preferably, thevolatile distillate fraction will have an ASTM Color of less than orequal to 1.5.

The hydrodesulfurization catalyst can include, but is not limited to, atleast a first metal component selected from Groups 8-10 (IUPAC) metalssuch as iron, cobalt, and/or nickel, and at least a second metalcomponent selected from Group 6 (IUPAC) metals such as molybdenum and/ortungsten, on a high surface area support material such as alumina. Othersuitable desulfurization catalysts include zeolitic catalyts as well asnobel metal catalyts where the noble metal is palladium or platinum.More than one type of hydrodesulfurization catalyst can be used in thesame reaction vessel or zone to remove sulfur. In a preferredembodiment, the hydrodesulfurization catalyst is a nickel molybdenumcatalyst.

In a preferred embodiment, the hydrodesulfurization conditions are thefollowing operation conditions:

Temperature: 200-485 degrees Celsius

Pressure: 8-200 bar

Liquid Hourly Space Velocity: 0.1-10 hr⁻¹

With consideration of FIG. 1, the flow of a process according to thepresent invention includes providing a feedstock of petroleum crude oilvia a line 10 into a main reactor 20 wherein hydrogen and ahydrodesulfurization catalyst are provided. The main reactor 20 may be ahydrotreater or other reactor as known in the art. Underhydrodesulfurization conditions, the sulfur-containing components of thefeedstock and the hydrogen react.

The reaction products flow through line 25 into the product fractionator30, wherein products are fractionated into a naptha fraction 40, akerosene fraction 50, and a first bottoms fraction 60. The napthafraction 40 is collected via line 45. The kerosene fraction is collectedvia line 55. For convenience, line 45 and line 55 may be a single line.

The first bottoms fraction 60 flows through line 65 into a flashdistillation column or flash tank 70 to separate the volatile distillatefraction 80 from a second bottoms fraction 90 containing heavy diesel,sulfur, colorants, and other impurities. Depending upon the stageefficiency of flash distillation column 70, the volatile distillatefraction 80 may contain at least about 80% of the first bottoms stream'sdiesel having an ASTM Color of less than or equal to 2.5 and a sulfurcontent of less than 15 ppm. The corresponding second bottoms fraction90 may contain up to about 20% of the first bottoms stream's heavydiesel.

Preferably, the the distillation column for the flash distilling stepwill have a stage efficiency in the range of about 80% to about 99%.More preferably, the distillation column for the flash distilling stepwill have a stage efficiency of about 95%.

EXAMPLES

The following non-limiting examples illustrate the invention.

Samples of a first bottoms fraction were collected from ahydrodesulfurizing manufacturing plant. Each sample was evaluated forinitial ASTM Color and post-treatment ASTM Color according to ASTMD6045, the contents of which are incorporated herein by reference in itsentirety.

The post-treatment methods were either (a) flash distilling the samplewith a single-stage flash distillation column to 99% efficiency (i.e.,99% volatile distillate fraction and 1% bottoms fraction) or (b)reacting the sample with hydrogen in the presence of ahydrodesulfurization catalyst under hydrodesulfurization conditions in apilot plant hydrotreater. The flash distillation method demonstratedthat the volatile distillate fraction satisfies the ASTM Colorrequirement, and the additional hydrotreatment method showed conversionof the sample into ULSD that satisfies the ASTM Color specification.

Flash Distillation (Comparative Examples 1 and 2)

Example No. Initial ASTM Color Post-Treatment ASTM Color 1 5.2 1.1 2 3.90.3

Additional Hydrotreatment (Examples 3 and 4)

Example No. Initial ASTM Color Post-Treatment ASTM Color 3 3.3 1.7 4 4.81.9

Although the invention has been described in considerable detail by thepreceding specification, this detail is for the purpose of illustrationand is not to be construed as a limitation upon the following appendedclaims. All cited ASTM standards, reports, references, U.S. patents,allowed U.S. patent applications, and U.S. Patent ApplicationsPublications are incorporated herein by reference.

What is claimed is:
 1. A process for preparing ultra low sulfur dieselcomprising the steps of: (a) reacting a feedstock of petroleum crude oilwith hydrogen in the presence of a hydrodesulfurization catalyst underhydrodesulfurization conditions; (b) fractionating the reaction productsinto a naphtha fraction, a kerosene fraction, and a first bottomsfraction; (c) flash distilling the first bottoms fraction in adistillation column having at least one stage and a total stageefficiency of less than about 99%, thereby yielding (i) a volatiledistillate fraction having an ASTM Color of less than or equal to 2.5and a sulfur content less than 15 ppm and (ii) a second bottomsfraction; (d) condensing the volatile distillate fraction as ultra lowsulfur diesel; and (e) recycling the second bottoms fraction into thefeedstock of petroleum crude oil for further reacting of the secondbottoms fraction with hydrogen in the presence of thehydrodesulfurization catalyst.
 2. The process of claim 1 wherein thereacting step occurs in a hydrotreater.
 3. The process of claim 1wherein the distillation column for the flash distilling step has astage efficiency of at least about 80%.
 4. The process of claim 3wherein the distillation column for the flash distilling step has astage efficiency in the range of about 80% to about 99%.
 5. The processof claim 4 wherein the distillation column for the flash distilling stephas a stage efficiency of about 95%.
 6. The process of claim 1 whereinthe hydrodesulfurization catalyst is a zeolite catalyst.
 7. The processof claim 1 wherein the hydrodesulfurization catalyst is a nickelmolybdenum catalyst.
 8. The process of claim 1 wherein thehydrodesulfurization catalyst consists of more than onehydrodesulfurization catalyst.
 9. The process of claim 1 wherein thehydrodesulfurization catalyst comprises palladium.
 10. The process ofclaim 1 wherein the hydrodesulfurization catalyst comprises platinum.11. The process of claim 1 wherein the hydrodesulfurization catalystcomprises at least a first metal component selected from Group 8, 9, and10 (IUPAC) metals and at least a second metal component selected fromGroup 6 (IUPAC) metals, on a high surface area support material.
 12. Theprocess of claim 11 wherein the first metal component is selected fromthe group consisting of iron, cobalt, and nickel.
 13. The process ofclaim 11 wherein the second metal component is selected from the groupconsisting of molybdenum and tungsten.
 14. The process of claim 11wherein the high surface area support material is alumina.
 15. Theprocess of claim 11 wherein the reacting step occurs at a temperature of200-485 degrees Celsius.
 16. The process of claim 11 wherein thereacting step occurs at a pressure of 8-200 bar.
 17. The process ofclaim 11 wherein the reaction has a liquid hourly space velocity of0.1-10 hr⁻¹.
 18. A process for preparing ultra low sulfur dieselcomprising the steps of: (a) reacting a feedstock of petroleum crude oilwith hydrogen in a hydrotreater in the presence of a nickel molybdenumhydrodesulfurization catalyst under hydrodesulfurization conditions of(i) a temperature of 200-485 degrees Celsius, (ii) a pressure of 8-200bar, and (iii) a liquid hourly space velocity of 0.1-10 hr⁻¹; (b)fractionating the reaction products into a naphtha fraction, a kerosenefraction, and a first bottoms fraction; (c) flash distilling the firstbottoms fraction in a distillation column having at least one stage anda total stage efficiency of less than about 99%, thereby yielding (i) avolatile distillate fraction having an ASTM Color of less than or equalto 2.5 and a sulfur content less than 15 ppm and (ii) a second bottomsfraction; (d) condensing the volatile distillate fraction as ultra lowsulfur diesel; and (e) recycling the second bottoms fraction into thefeedstock of petroleum crude oil for further reacting of the secondbottoms fraction with hydrogen in the presence of the nickel molybdenumhydrodesulfurization catalyst.